Valve has announced today as part of the Uber Update that Team Fortress 2 is now free to play forever. As of right now, everyone can download TF2 at no charge and play all game modes, maps, and classes. There will be two tiers of account: free and premium.
All accounts that download the game for free will receive a free account, with 50 backpack slots, standard items, limited crafting blueprints, and the ability to receive items and gifts.
Anyone who has spent any type of money on Team Fortress 2, whether buying the game or making an in-game purchase, will receive a premium account. This expands their backpack to 300 slots (expandable to 1,000), rare and cosmetic items in addition to the standard ones, all crafting blueprints, full trading, and the ability to both receive and give gifts.
In addition, all players who have purchased the game through Steam, or who purchase the game at retail, will receive a free Proof of Purchase hat. Valve will also tighten up account monitoring to ensure that players can’t cheat the system and make infinite free accounts to, for example, bypass bans.
Source: Team Fortress 2 website